In electronic devices such as a portable telephone, various LEDs (Lighting Emitting Diode) are used as backlights of a liquid crystal display (LCD) or as user interfaces. In addition to simple lighting of LEDs, light emitting intensity or state of flickering of LEDs may be changed dependent on the state of the device or by user setting. Generally, such visual effects are realized by arranging three types of LEDs, that is, “red”, “green” and “blue”, and by adjusting respective light emitting intensities.
The light emitting intensity of an LED changes dependent on the amount of supplied current and, therefore, in order to represent an intended color, a current control circuit controlling a value of current to be supplied to each LED becomes necessary. In this regard, Japanese Patent Laying-Open No. 2002-111786 (Patent Document 1), for example, discloses a light emitting device in which the value of current to be supplied is controlled by pulse width modulation (PWM, in the following, also simply denoted as PWM method).
In the PWM method, a pulse signal having H (High) and L (Low) levels appear at every prescribed period, and a current is supplied to a load only in the time period when the pulse signal is at the H-level. Therefore, the longer the H-level time period, the larger the supplied current becomes. In the PWM method, the ratio of H-level time period in one period, that is, the duty ratio, is changed to control the value of current to be supplied.
Further, in order to exhibit higher visual effects, proposals have been made to change the light emitting intensity or the overall tone in accordance with user settings or signals from a sensor or sensors. By way of example, Japanese Patent Laying-Open No. 2004-205669 (Patent Document 2) discloses an LED control circuit in which light emitting intensity is changed in accordance with a sound pressure level of the voice.    Patent Document 1: Japanese Patent Laying-Open No. 2002-111786    Patent Document 2: Japanese Patent Laying-Open No. 2004-205669